1. Field of the Invention
The present invention relates to a DC—DC converter which forms, for example, a boosting (stepup) or bucking (stepdown) circuit that increases or decreases a direct-current voltage.
2. Description of the Related Art
FIG. 28 shows an example of a conventional boosting circuit, which is composed of an oscillator (OSC) 1, a pump circuit (PMP) 2, a voltage detecting circuit 3 consisting of resistors R1 and R2, and a comparator (CMP) 4. The oscillator 1 produces a pulse signal. The pump circuit 2, which is formed of a capacitor and a charge-transfer diode or a capacitor and a charge-transfer transistor, produces a boosted voltage in response to the output signal of the oscillator 1. The voltage detecting circuit 3 detects the output voltage of the pump circuit 2. The comparator 4, which is comprised of a differential amplifier by way of example, makes a comparison between the output voltage of the voltage detecting circuit 3 and a reference voltage Vref and then outputs a signal corresponding to the difference therebetween. When the output voltage of the voltage detecting circuit 3 is lower than the reference voltage Vref, the comparator 4 enables the oscillator 1. When the output voltage of the voltage detector 3 is higher than the reference voltage Vref, that is, when the boosted voltage has reached the target voltage, the comparator 4 disables the oscillator 1.
As an invention relating to this type of boosting circuit, a circuit is known which is capable of producing a high voltage with low power dissipation (see, for example, Unexamined Japanese Patent Publication No. 10-302492).
With the comparator comprised of a differential amplifier, its output response is slow when the bias current is low. Increasing the bias current in order to make the response faster results in increased current dissipation. For this reason, it is not desirable to supply too high a bias current. Consequently, the conventional comparator is used in a state where its output response is slow. Thus, it is impossible for the comparator which is slow in response to stop the operation of the boosting circuit as soon as its output voltage has reached the target voltage. The pump circuit 2 would therefore be shut down after several pumping operations. For this reason, the boosted voltage would increase above the target voltage, causing a ripple. In general, a decoupling capacitor is connected between the output of the pump circuit 2 and ground potential in order to reduce the ripple. To reduce the ripple, however, a capacitor of large capacitance would be needed. Thus, the method for reducing the ripple using a capacitor involves a problem that the chip size increases.
If the pump circuit 2 should be disabled in the middle of an operation, charges may remain on the capacitor that forms the pump circuit or they may flow backward. These charges may cause noise. It is therefore required to disable the pump circuit precisely. The above problems were described in terms of a boosting circuit; however, the same applies to a bucking circuit.
Accordingly, there is a demand for a DC—DC converter that is capable of reducing the ripple in the output voltage and preventing the generation of noise.